The invention relates to an arrangement in a rock drill comprising at its front end a reciprocating shank to be impacted by a percussion piston reciprocating in the travel direction of the shank, and lifting means, driven by pressurized fluid, for moving the shank toward the percussion piston.
The invention further relates to a method of controlling rock drilling upon drilling a downward extending hole by extension rod drilling by a rock drill comprising at its front end a reciprocating shank to which an extension rod is secured and which is impacted by a percussion piston reciprocating in the travel direction of the shank, a lifting sleeve surrounding the shank and comprising on the side of the percussion piston a lifting surface acting on the shank, at least two lifting pistons arranged to act on the shank and serving to lift the shank by means of pressurized fluid pressure toward the percussion piston so as to move the shank to a desired percussion point in the longitudinal direction of the rock drill, a feed force pushing the rock drill forward being arranged to act in the rock drill during drilling.
In some cases the problem in rock drills is that it should be possible to lift the shank to the percussion point at the moment when the drill equipment gets jammed. This is typically implemented by arranging what is known as a lifting piston either to the shank or separately around it. The pressurized fluid pressure, set to act on the lifting piston, serves to move the shank toward the percussion piston with respect to the front end of the rock drill. Such solutions are known from e.g. U.S. Pat. Nos. 4,109,734, 4,718,500, and 5,002,136. In these solutions the shank is encased in a separate lifting piston, which is a separate annular piece around the shank. The lifting piston moves in a cylinder chamber arranged to the drill body, and has to be sealed in the chamber on both sides in order for the pressurized fluid fed into the cylinder chamber to push the lifting piston, and consequently, the shank through a support surface in the shank, toward the percussion piston, and consequently, the percussion point. The problem in these solutions is that to seal a piston is cumbersome and similarly the clearances have to be relatively wide in every respect in order for the lifting piston and the shank to be able to settle in a suitable position with respect to each other even when the shank is loaded. This again results in extensive pressurized fluid leakage and, consequently, increases pressurized fluid consumption. Furthermore, to seal this structure reliably is cumbersome, and sealing damages occur easily, causing extra operational and maintenance costs.
U.S. Pat. No. 4,582,145 again discloses a solution in which a separate lifting piston surface, moving in a cylinder in the drill body, is arranged to the shank. In this embodiment the shank can be lifted by feeding pressurized fluid into the cylinder chamber so as to make the pressure act on the piston surface of the shank and thus move the shank towards its percussion point. The lifting piston has to be sealed carefully even in this structure and manufacturing the shank causes extra costs. Similarly, the front end of the shank has to be mounted on bearings and sealed in such a manner that when the shank exerts a forward impact, the pressurized fluid being discharged from the cylinder chamber does not break the seals at the front end. This increases the requirements set on the entire structure, and naturally results in increased manufacturing costs.
Another problem in all these solutions is that the lifting force of the shank can only be adjusted or controlled by adjusting the pressurized fluid pressure, and as a result the force of the lifting piston may cause an unnecessarily high resistance to the impact movement generated by the percussion piston. This again causes waste of capacity and unnecessary heating of the pressurized fluid, resulting in lower total drilling capacity.
It is the object of the present invention to provide an arrangement for implementing the lifting of the shank to impact position simply and easily and for selecting the lifting force, which acts on the shank, suitably according to the circumstances. It is a further object of the invention to provide an arrangement that is easy and simple to manufacture and that operates reliably and safely.
It is still a further object of the present invention to provide a method of controlling rock drilling easily and simply when drilling a downward extending hole particularly by extension rod drilling so as to be able to maintain given drilling adjustment parameters substantially the same irrespective of the number of extension rods or the weight of the drill rod, and with which the drilling capacity can be adjusted in various ways as need be, according to the circumstances.
The arrangement of the invention is characterized in that the lifting means comprise around the shank a lifting sleeve comprising on the side of the percussion piston a lifting surface acting on the shank, around the shank a plurality of cylindrical lifting pistons acting on the lifting sleeve at one end and comprising a cylinder chamber for pressurized fluid at the opposite end.
The method of the invention is further characterized in that upon downward drilling, the magnitude of the feed force is reduced as the number of extension rods increases proportionately to their weight, and that as the feed force reaches a preset threshold value Fmin, such a pressurized fluid pressure is set to act in at least some lifting pistons that will maintain the force acting between the rock drill and the shank and moving the rock drill body towards the shank to a desired percussion point substantially at said value.
It is an essential idea of the arrangement of the invention that the shank is lifted by using a separate lifting sleeve which itself does not operate as a piston but instead only transmits the lifting force to the shank. It is a further essential idea of the invention that for generating the lifting force, at least two cylindrical pistons that are placed around the shank substantially symmetrically and that are disposed each in a dedicated cylinder chamber, possibly suitably sealed. It is characteristic of a preferred embodiment of the invention that it comprises at least two groups of pistons with different travel lengths and by means of which the shank can be lifted, depending on the circumstances, a different length towards the percussion point.
It is an essential idea of the method of the invention that when the feed force acting on the rock drill is being reduced as the weight of the extension rods increases, a sufficient power transmission can be ensured by feeding behind the lifting pistons, when required, a pressure which will maintain the force moving the rock drill and the shank toward each other at the level of a preset force, resulting in the shank being at the desired percussion point while a sufficient impact energy transmission is also achieved from the percussion piston via the shank to the extension rod. This way other drilling parameters can be kept in a desired manner substantially the same irrespective of how many extension rods or what kind of a drill rod is secured to the rock drill.
It is an advantage of the invention that the lifting sleeve does not actually need any seal, making its manufacture and mounting easy. It is a further advantage of the invention that the manufacture of small piston cylinders for the drill body or a piece to be secured to the body, and similarly the manufacture of small cylindrical pistons, is easy and simple compared with known solutions. It is still a further advantage of the invention that to control the lifting is easy and simple to implement for different lifting travel lengths. It is still an advantage of the invention that it is easy to add the structure of the invention to existing machines by minimal change of parts. It is a further advantage of the invention that the clearances of the pistons having small diameters are also small, and consequently leakage is minimal, the pistons not necessarily having to be sealed because the leaked oil can be used for lubrication. Still a further advantage is that since the normal structure of the rock drill does not increase in length since the pistons, the shank bearing and a possible flushing device can be incorporated into the same structure even at substantially the same axial point. A further advantage is that when the pistons comprise a seal, the small clearance of the piston throttles the pressure pulse produced by an impact in the seal minimizing the risk of damage to the seal. It is an advantage of the method of the invention that the drilling is easy to adjust since the values of the adjustment parameters needed for adjusting the actual drilling do not have to be changed as the weight of the drill rod or extension rod changes, but the change in weight can be compensated for by means of the feed force and the pressurized fluid pressure acting behind the lifting pistons. A further advantage is that the magnitude of the percussion force acting via the drill bit on the rock to be drilled can be adjusted by adjusting the value of the pressure behind the lifting pistons in such a manner that the shank remains a distance towards the front end of the rock drill from its optimal percussion point, the percussion piston impacting partly on its damping cushion and part of the percussion force being damped at the same time as only the remaining part is able to move via the shank to the drill rod and thus to the drill bit.
The invention will be described below in greater detail in connection with the accompanying drawings.